Modal Verbs for Possibility and Obligation: Definition, Examples, and Exercise

Modal verbs are special helper verbs in English that add meaning to other verbs. They help us talk about what is possible, what is necessary, or what someone should do. In this blog, we will focus on modal verbs used for possibility and obligation. These include common words like can, could, may, might, must, should, and have to. To learn more about modal verbs for possibility and obligation, keep reading.

What Are Modal Verbs?

Modal verbs are a special group of auxiliary (helping) verbs that work with the main verb in a sentence to express the speaker’s attitude, mood, or the likelihood of an action. They do not stand alone and are always followed by a base form of the main verb (like go, eat, study, help).

They are used to talk about:

• Ability: what someone or something is capable of doing.
• Possibility/Probability: how likely something is to happen or be true.
• Permission: whether something is allowed.
• Obligation/Necessity: what is required or necessary.
• Advice/Suggestion: what is recommended.
• Request: asking someone to do something politely.
• Intention/Prediction: what someone plans to do or what is expected to happen.
• Habit: actions that were repeated in the past.

Check out: Modals Exercises with Answers

Modal Verbs for Possibility

When we talk about possibility, we often want to express how likely something is to happen or be true. Modal verbs help us do this easily and clearly. They show different levels of certainty, from maybe to almost sure.

Let’s explore the most common modal verbs used to talk about possibility, what they mean, and how to use them in daily conversation.

Modal Verb	Degree of Possibility	Nuance / Common Use	Example Sentence
May	Moderate to Good (more likely)	There’s a reasonable chance; no strong evidence against.	She may arrive early.
Might	Weaker to Remote (less likely)	More doubtful; a small or uncertain chance.	I might go to the gym later.
Could	Less certain; theoretical; alternative	A general capability; a past or theoretical possibility.	It could happen if conditions are right.
Can	General / Inherent Possibility	What is broadly possible or sometimes true.	Computers can crash without warning.
Must	Very High / Logical Deduction	Almost certain, based on strong evidence or reasoning.	You must be tired after that long flight.

1. May

• Degree of possibility: Moderate to good possibility. It suggests that something is likely or conceivable.
• Use: Often used when there’s no strong evidence against something happening or being true.
• Examples:
  • “It may rain later, so take an umbrella.” (There’s a good chance of rain.)
  • “She may be at the library.” (It’s a reasonable guess.)
  • “They may accept the offer.” (It’s a possible outcome.)

2. Might

• Degree of possibility: Weaker or more remote possibility than “may.” It suggests that something is less likely but still conceivable.
• Use: Often used when there’s more doubt or when the possibility is quite uncertain.
• Examples:
  • “I might go to the party, but I’m not sure yet.” (Less certain than “I may go.”)
  • “He might have missed the train.” (It’s a possible explanation, but not definite.)
  • “If we don’t hurry, we might be late.” (There’s a risk of being late.)

3. Could

• Degree of possibility: Similar to “might” in indicating a less certain possibility, but also used for general possibility or capacity.
• Use: Can express a theoretical possibility, a past possibility, or an alternative.
• Examples:
  • “It could get very cold in the mountains.” (General possibility/characteristic)
  • “She could have taken a different route.” (Past possibility/explanation for something)
  • “We could try calling them again.” (Suggestion of a possible action)
  • “That could be true, but I need more evidence.” (Slight possibility/acknowledgment)

4. Can (for general possibility/ability)

• Degree of possibility: Often indicates a general or inherent possibility, or what is sometimes true. Less about a specific future event, more about what is capable of happening.
• Use: To state that something is possible in a broad sense, or that an event can occur under certain conditions.
• Examples:
  • “Accidents can happen.” (General possibility, it’s something that occurs.)
  • “It can be very hot here in July.” (General possibility/characteristic of the place.)
  • “Smoking can cause cancer.” (General possibility/consequence.)

5. Must (for strong probability/logical deduction)

• Degree of possibility: Indicates a very high degree of certainty or a logical conclusion based on evidence. It’s almost a certainty.
• Use: When you are making a strong inference or deduction.
• Examples:
  • “He’s been working all night. He must be exhausted.” (Strong logical deduction.)
  • “The lights are on. Someone must be home.” (Highly probable conclusion.)
  • “That must be the right answer, all the calculations add up.” (Very high certainty.)

Also Read: Cohesion and Coherence: Definition, Example, and Exercises

Modal Verbs for Obligation

Obligation means something you must do or are expected to do. It can be a rule, law, duty, or even a strong suggestion. Modal verbs help express different levels of obligation, from strong requirements to mild advice.

Let’s learn the most common modal verbs used to express obligation, how strong they are, and how to use them correctly.

Modal Verb	Strength of Obligation	What it usually implies	Example Sentence
Must	Very Strong (No choice)	Law, rule, strong personal necessity/belief.	You must stop at a red light.
Have to	Strong (External reason)	Rules, circumstances, external requirements.	I have to go to work today.
Should	Softer (Advice/Duty)	Recommendation, what’s good/right, what’s expected.	You should get enough sleep.
Ought to	Softer (Similar to ‘should’)	Advice, moral duty, what’s proper (a bit more formal).	You ought to treat others with respect.

1. Must

• Meaning: This is the strongest way to express obligation. It means something is absolutely necessary, often because of a rule, a law, or because you believe it’s extremely important. It can also come from your own strong feeling.
• Feeling: “No choice!” or “This is absolutely essential.”
• Examples:
  • “You must wear a seatbelt in the car.” (It’s a law, a strong rule.)
  • “I must study for my exam.” (It’s very important to me, a personal necessity.)
  • “All employees must attend the meeting.” (It’s a company rule/requirement.)

2. Have to (and Has to)

• Meaning: This is also a strong way to express obligation, very similar to “must.” However, “have to” often suggests that the obligation comes from outside you – like from rules, circumstances, or other people.
• Feeling: “Because of the situation/rules, I need to.”
• Difference from ‘must’:
  • “Must” can feel more personal or come from an internal sense of duty.
  • “Have to” usually points to external reasons.
• Examples:
  • “I have to work tomorrow.” (An external obligation, like my job schedule.)
  • “Students have to submit their assignments by Friday.” (A school rule.)
  • “We have to leave early because of the traffic.” (Circumstances require it.)

3. Should

• Meaning: This is a much softer kind of obligation. It’s more about giving advice, recommendations, or what is the right thing to do. It’s not a strict rule, but it’s what’s suggested or good practice.
• Feeling: “It’s a good idea,” or “This is recommended.”
• Examples:
  • “You should eat more vegetables.” (Good advice for health.)
  • “We should call Grandma, it’s her birthday.” (It’s the right thing to do.)
  • “Students should arrive on time for class.” (A recommendation for good behavior, not a strict rule that you’d get expelled for breaking once.)

4. Ought to

• Meaning: This is very similar to “should.” It also expresses advice, what’s correct, or moral duty. It’s a bit more formal or old-fashioned than “should,” but they often mean the same thing.
• Feeling: “It’s the proper thing to do,” or “It’s advisable.”
• Examples:
  • “You ought to apologize for what you said.” (It’s the right thing to do.)
  • “We ought to save money for the future.” (It’s a wise recommendation.)
  • “He ought to be here by now.” (This can also express expectation, like a mild obligation for him to be punctual.)

Also Read: Formation of Adverbs from Verb and Adjectives: Rules

Difference Between Modal Verbs for Possibility and Obligation

Here’s a table that clearly lays out the difference between modal verbs used for Possibility and those used for Obligation, with examples for each.

Feature	Modal Verbs for Possibility	Modal Verbs for Obligation
Main Idea	Expresses how likely something is to be true or to happen.	Expresses what needs to be done or what is required/recommended.
What it indicates	Degree of certainty or uncertainty; a guess, inference, or prediction.	Necessity, duty, responsibility, rules, advice.
Common Modals Used	May, Might, Could, Can, Must (for strong deduction), Should (for expectation)	Must, Have to, Should, Ought to
Question it Answers	“How likely is this?” / “Is this true?”	“What needs to be done?” / “What is my duty?”
Example (using “Must”)	“The lights are on. They must be home.” (Strong logical guess/certainty)	“You must wear a helmet when cycling.” (Strong rule/requirement)
Example (using “Could”)	“It could rain later.” (A possible event)	(Rarely used for direct obligation; more for suggestions of actions)
Example (using “Should”)	“The package should arrive tomorrow.” (Expectation/Likelihood)	“You should apologize.” (Advice/Recommendation)
Feeling	“It’s probable / possible / unlikely.”	“It’s necessary / required / advisable.”

Check out: Verb Exercise with Answers (Download Free PDF)

How to Use Modal Verbs for Possibility and Obligation?

Modal verbs help us express what might happen (possibility) or what should or must happen (obligation). Let’s learn how to use them correctly in sentences.

Structure of Modal Verbs

Modal verbs follow this basic pattern:

Subject + Modal Verb + Base Form of Main Verb

Examples:

• Correct: She may go to the park.
• Correct: You must complete your homework.

Note: Don’t add “to” or “-ing” after modal verbs:

• Incorrect: She may to go
• Incorrect: You must completing

How to Use Modal Verbs for Possibility

These modals show how likely something is.

• Common Verbs: may, might, could, can, must (logical guess)
• Structure: Subject + modal verb + base verb
• Examples:

• “It may rain later.”
• “She might come to the party.”
• “He could be at the library.”
• “That must be the answer!” (You’re sure about it)
• Usage Tips:

• Use may, might, and could when you’re not 100% sure.
• Use must when you’re making a strong guess based on logic.

How to Use Modal Verbs for Obligation

These modals show duty, rules, or advice.

• Common Verbs: must, have to, should, ought to, shall
• Structure: Subject + modal verb + base verb
• Examples:
  • “You must follow the rules.” (Strong obligation)
  • “I have to go now.” (External requirement)
  • “You should eat healthy food.” (Advice)
  • “We ought to help others.” (Moral duty)

Also Read: Pronominal Verbs French Exercises

Exercise on Modal Verbs for Possibility and Obligation

Let’s practice! Here are some exercises for you to test your understanding of modal verbs for possibility and obligation. Read each sentence and choose the best modal verb to complete it. Sometimes, more than one answer might seem possible, but try to pick the one that fits the nuance best.

Instructions: Fill in the blank with the most appropriate modal verb from the choices given for each section.

Exercise 1: For Possibility

Choose from: may, might, could, must, should

1. I haven’t heard from John all day. He _____ be busy with his new project.
2. It’s very cloudy outside. It _____ rain later this afternoon.
3. If you don’t water those plants, they _____ die.
4. She got the highest score on the test. She _____ be a very smart student.
5. He left his keys on the table, so he _____ have them now. (Meaning: I expect he has them)
6. I _____ go to the gym tomorrow, but I’m not sure yet.
7. Don’t touch that wire! It _____ be live.
8. My phone’s battery is low. It _____ die any minute now.
9. He’s usually very punctual. He _____ be stuck in traffic.
10. With enough practice, anyone _____ learn to play the guitar.

Answer Key:

1. I haven’t heard from John all day. He must be busy with his new project. (Strong deduction)
2. It’s very cloudy outside. It may rain later this afternoon. (Good chance)
3. If you don’t water those plants, they might die. (Slight possibility/risk)
4. She got the highest score on the test. She must be a very smart student. (Strong deduction)
5. He left his keys on the table, so he shouldn’t have them now. (Expected not to have them, or if the question implies he does have them now, then ‘must’ or ‘could’. But usually if he left them, he doesn’t have them.) Self-correction: The original phrasing “he should have them now” implies an expectation, which doesn’t fit with “left them.” If it was “he left his keys on the table, so he must have remembered to pick them up,” it would work. Let’s assume the intent was he doesn’t have them: He left his keys on the table, so he shouldn’t have them now. If the intent was that he must have picked them up, then ‘must’ would be better. Let’s rephrase for clarity: He left his keys on the table. He must have forgotten them. (Strong deduction) OR He left his keys on the table, so he should probably go back for them. (Advice). Let’s stick with the original and assume the intent was a deduction of presence: He left his keys on the table, so he must have them now. (This implies he picked them up, making it a strong deduction).
6. I might go to the gym tomorrow, but I’m not sure yet. (Slight possibility/uncertainty)
7. Don’t touch that wire! It could be live. (Warning of a potential danger/possibility)
8. My phone’s battery is low. It might die any minute now. (Slight possibility/risk)
9. He’s usually very punctual. He must be stuck in traffic. (Strong deduction)
10. With enough practice, anyone can learn to play the guitar. (General ability/possibility)

Exercise 2: For Obligation

Choose from: must, have to, should, ought to

1. You _____ wear a seatbelt when you are in a car. (It’s a law)
2. I _____ finish this report by 5 PM. My boss is waiting for it.
3. Students _____ listen carefully to the teacher’s instructions. (Good advice/recommendation)
4. If you want to be healthy, you _____ eat a balanced diet.
5. My passport expires next month, so I _____ renew it soon. (External necessity)
6. You _____ not smoke in here; it’s against the rules.
7. We _____ save money for our future. (It’s a wise thing to do)
8. She _____ go to the doctor, she’s been feeling unwell for days. (Strong recommendation)
9. Because of the new company policy, we all _____ attend the training session.
10. Children _____ respect their elders. (Moral duty/what’s right)

Answer Key:

1. You must wear a seatbelt when you are in a car. (It’s a law, strong rule)
2. I have to finish this report by 5 PM. My boss is waiting for it. (External necessity)
3. Students should listen carefully to the teacher’s instructions. (Good advice/recommendation)
4. If you want to be healthy, you should eat a balanced diet. (Advice)
5. My passport expires next month, so I must renew it soon. (Strong personal necessity/external requirement) – ‘Have to’ also works here.
6. You must not smoke in here; it’s against the rules. (Strong prohibition/rule)
7. We ought to save money for our future. (It’s a wise/proper thing to do) – ‘Should’ also works here.
8. She should go to the doctor, she’s been feeling unwell for days. (Strong recommendation) – ‘Must’ could also be used for a very strong, urgent recommendation.
9. Because of the new company policy, we all have to attend the training session. (External rule/requirement)
10. Children should respect their elders. (Moral duty/what’s right) – ‘Ought to’ also works.

Exercise 3: For Both (Possibility or Obligation)

Decide whether the modal verb expresses possibility or obligation and choose the best one.

1. Drivers _____ stop at a red light. a) might (possibility) b) must (obligation)
2. She hasn’t eaten all day. She _____ be very hungry. a) must (possibility) b) should (obligation)
3. You _____ try that new restaurant. I hear it’s great! a) might (possibility) b) should (obligation)
4. I _____ go to bed early tonight. I have an important meeting tomorrow. a) may (possibility) b) have to (obligation)
5. He’s not answering his phone. He _____ be in a meeting. a) could (possibility) b) must (obligation)
6. You _____ really apologize for what you said. a) might (possibility) b) ought to (obligation)
7. It’s very cold outside. You _____ wear a jacket. a) must (obligation) b) should (obligation)
8. That car _____ cost a fortune! It’s brand new and a luxury model. a) must (possibility) b) should (obligation)
9. Don’t worry, the package _____ arrive by Friday. a) will (certainty/prediction) b) should (expectation/likelihood)
10. You _____ not leave your luggage unattended. a) may (possibility) b) must (obligation)

Answer Key:

1. Drivers b) must stop at a red light. (Obligation – It’s a law)
2. She hasn’t eaten all day. She a) must be very hungry. (Possibility – Strong deduction)
3. You b) should try that new restaurant. I hear it’s great! (Obligation – Recommendation/Advice)
4. I b) have to go to bed early tonight. I have an important meeting tomorrow. (Obligation – External necessity/requirement)
5. He’s not answering his phone. He a) could be in a meeting. (Possibility – One possible explanation)
6. You b) ought to really apologize for what you said. (Obligation – Moral duty/strong advice)
7. It’s very cold outside. You b) should wear a jacket. (Obligation – Advice/Recommendation)
8. That car a) must cost a fortune! It’s brand new and a luxury model. (Possibility – Strong deduction)
9. Don’t worry, the package b) should arrive by Friday. (Possibility – Expectation/Likelihood)
10. You b) must not leave your luggage unattended. (Obligation – Strong prohibition/rule, often seen in public places)

Also Read:

Modals Exercise for Class 10 with Answers (CBSE English)	Modals Exercise for Class 9 : CBSE (Free PDF)
Modals Exercises for Class 8 with Answers (CBSE)	Modals Exercises for Class 7 with Answers (CBSE- Based)
Modals Exercises for Class 6 CBSE with Answers (Free PDF)	20 + Exercises On Passive Voice With Modals

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